LTE Physical Structure
3rd Generation Project Partnership (3GPP) Long Term Evolution (LTE) supports a type 1 radio frame structure, which is applicable to Frequency Division Duplex (FDD), and a type 2 radio frame structure, which is applicable to Time Division Duplex (TDD).
FIG. 1 shows the structure of a type 1 radio frame. The type 1 radio frame includes 10 subframes, each of which consists of two slots.
FIG. 2 shows the structure of a type 2 radio frame. The type 2 radio frame includes two half-frames, each of which is composed of five subframes, a Downlink Pilot Time Slot (DwPTS), a Guard Period (GP), and an Uplink Pilot Time Slot (UpPTS), in which one subframe consists of two slots. The DwPTS is used for initial cell search, synchronization, and channel estimation at a User Equipment (UE). The UpPTS is used for channel estimation and uplink transmission synchronization of the UE at a Base Station (BS). The GP is used to remove interference occurring in uplink due to a multipath delay of a downlink signal between the uplink and a downlink. Meanwhile, one subframe consists of two slots regardless of the radio frame type.
FIG. 3 shows the structure of an LTE downlink slot. As shown in FIG. 3, a signal transmitted in each slot can be described by a resource grid including NRBDL NSCRB subcarriers and NsymbDL Orthogonal Frequency Division Multiplexing (OFDM) symbols. Here, NRBDL represents the number of Resource Blocks (RBs) in downlink, NSCRB represents the number of subcarriers constituting one RB, and NsymbDL represents the number of OFDM symbols in one downlink slot.
FIG. 4 shows the structure of an LTE uplink slot. As shown in FIG. 8,
a signal transmitted in each slot can be described by a resource grid including NRBUL NSCRB subcarriers and NsymbUL OFDM symbols. Here, NRBUL represents the number of RBs in uplink, NSCRB represents the number of subcarriers constituting one RB, and NsymbUL represents the number of OFDM symbols in one uplink slot.
A Resource Element (RE) is a resource unit defined as an index (a, b) in the uplink slot and the downlink slot and represents one subcarrier and one OFDM symbol. Here, ‘a’ is an index on a frequency axis and ‘b’ is an index on a time axis.
FIG. 5 shows the structure of a downlink subframe. In FIG. 5, a maximum of three OFDM symbols located at a front portion of a first slot within one subframe corresponds to a control region allocated to a control channel. The other OFDM symbols correspond to a data region allocated to a Physical Downlink Shared Channel (PDSCH). Examples of downlink control channels used in 3GPP LTE include a Physical Control Format Indicator Channel (PCFICH), a Physical Downlink Control Channel (PDCCH), a Physical Hybrid automatic repeat request Indicator Channel (PHICH), etc.
Definition of MIMO Technology
MIMO is an abbreviation for Multiple-Input Multiple-Output and refers to a method of improving data transmission/reception efficiency using multiple transmission antennas and multiple reception antennas, instead of a conventional method employing one transmission antenna and one reception antenna. In other words, MIMO technology allows a transmitter or a receiver of a wireless communication system to use multiple antennas, so that capacity or performance can be improved. Here, MIMO refers to multiple antennas.
In order to receive a message, MIMO technology is not dependent on a single antenna path. Instead, the MIMO technology applies a technique of completing the received whole message by gathering data fragments received via several antennas. Since the MIMO technology can improve a data transmission rate in a specific range or increase a system range at a specific data transmission rate, it is the next generation mobile communication technology widely usable for mobile communication terminals, relays, etc. Attention is being paid to this technology as a next-generation technology capable of overcoming limitations in mobile communication transmission capacity that has become a critical situation due to expansion of data communication.
FIG. 6 shows the configuration of a general MIMO communication system. As shown in FIG. 6, if the numbers of transmission and reception antennas are simultaneously increased to NT and NR, respectively, a theoretical channel transmission capacity is increased in proportion to the number of antennas, unlike the case where only either a transmitter or a receiver uses multiple antennas. Accordingly, it is possible to increase transmission rate and to remarkably improve frequency efficiency. Theoretically, the transmission rate according to an increase in channel transmission capacity can be increased by an amount obtained by multiplying an increase rate Ri indicated in the following Equation 1 by a maximum transmission rate Ro in case of using one antenna.Ri=min(NT,NR)  [Equation 1]
For example, in a MIMO communication system using four transmission antennas and four reception antennas, it is possible to theoretically obtain a transmission rate which is four times a transmission rate of a single antenna system. After an increase in the theoretical capacity of the MIMO system was first proved in the mid-1990s, various techniques for substantially improving data transmission rate have been actively developed. Several of these techniques have already been incorporated in a variety of wireless communication standards such as the 3rd generation mobile communication and the next-generation wireless local area network.
Active research up to now related to the MIMO technology has focused upon a number of different aspects, including research into information theory related to the computation of MIMO communication capacity in various channel environments and in multiple access environments, research into wireless channel measurement and model derivation of a MIMO system, and research into space-time signal processing technologies for improving transmission reliability and transmission rate.
UE-Specific Reference Signal Allocation Scheme in 3GPP LTE Downlink System
In the structure of the radio frame applicable to FDD out of the above-described radio frame structures supported by the 3GPP LTE, one frame is transmitted during a duration of 10 msec. One frame consists of 10 subframes, each of which has a duration of 1 msec. One subframe consists of 14 or 12 OFDM symbols. The number of subcarriers selected in one OFDM symbol can be one of 128, 256, 512, 1024, 1536, and 2048.
FIG. 7 shows the structure of a UE-specific downlink Reference Signal (RS) in a subframe using a normal Cyclic Prefix (CP) in which one Transmission Time Interval (TTI) has 14 OFDM symbols. In FIG. 7, ‘R5’ denotes a UE-specific RS and ‘l’ denotes a position of an OFDM symbol in a subframe.
FIG. 8 illustrates the structure of a UE-specific downlink RS in a subframe using an extended CP in which one TTI has 12 OFDM symbols.
FIGS. 9 to 11 show the structures of UE-common downlink RSs for systems having 1Tx, 2Tx, and 4Tx, respectively, when one TTI has 14 OFDM symbols. In FIGS. 9 to 11, R0, R1, R2, and R3 represent pilot symbols for transmission antenna port 0, transmission antenna port 1, transmission antenna port 2, and transmission antenna port 3, respectively. To eliminate interference with the other transmission antennas except for the transmission antennas transmitting the pilot symbols, no signals are transmitted in subcarriers where the pilot symbols of the respective transmission antennas are used.
The UE-specific downlink RSs shown in FIGS. 7 and 8 may be simultaneously used together with the UE-common downlink RSs shown in FIGS. 9 to 11. For example, the UE-common downlink RSs shown in FIGS. 9 to 11 may be used in OFDM symbols 0, 1, and 2 of a first slot in which control information is transmitted, and UE-specific downlink RSs may be used in the other OFDM symbols. If a predefined sequence (e.g. Pseudo-Random (PN) sequence, m-sequence, etc.) is multiplied by a downlink RS according to each cell before transmission, channel estimation performance in a receiver can be improved by reducing interference of a signal of a pilot symbol received from a neighboring cell. The PN sequence is applied in units of OFDM symbols in one subframe. Different PN sequences may be applied according to a cell ID, a subframe number, an OFDM symbol position, and a UE ID.
As an example, it can be understood that, in the structure of a pilot symbol in one transmission antenna (1Tx) shown in FIG. 9, two pilot symbols for 1Tx are used with respect to a specific OFDM symbol including pilot symbols. The 3GPP LTE system includes a variety of bandwidths ranging from 60 RBs to 110 RBs. Accordingly, the number of pilot symbols for one transmission antenna in one OFDM symbol including a pilot symbol is 2×NRB and a sequence multiplied by a downlink RS in each cell should have a length of 2×NRB. Here, NRB denotes the number of RBs corresponding to a bandwidth and the sequence may be a binary sequence or a complex sequence. One example of the complex sequence is indicated as r(m) in the following Equation 2.
                                          r            ⁡                          (              m              )                                =                                                    1                                  2                                            ⁢                              (                                  1                  -                                      2                    ·                                          c                      ⁡                                              (                                                  2                          ⁢                          m                                                )                                                                                            )                                      +                          j              ⁢                              1                                  2                                            ⁢                              (                                  1                  -                                      2                    ·                                          c                      ⁡                                              (                                                                              2                            ⁢                            m                                                    +                          1                                                )                                                                                            )                                                    ,                                  ⁢                  m          =          0                ,        1        ,        …        ⁢                                  ,                              2            ⁢                          N              RB                              ma                ⁢                                                                  ⁢                x                                              -          1                                    [                  Equation          ⁢                                          ⁢          2                ]            
In the above Equation 2, NRBmax represents the number of RBs corresponding to a maximum bandwidth and may be 110 according to the above description, and c represents a PN sequence and may be defined as a length-31 Gold sequence. In case of a UE-specific downlink RS, Equation 2 may be expressed by the following Equation 3.
                                          r            ⁡                          (              m              )                                =                                                    1                                  2                                            ⁢                              (                                  1                  -                                      2                    ·                                          c                      ⁡                                              (                                                  2                          ⁢                          m                                                )                                                                                            )                                      +                          j              ⁢                              1                                  2                                            ⁢                              (                                  1                  -                                      2                    ·                                          c                      ⁡                                              (                                                                              2                            ⁢                            m                                                    +                          1                                                )                                                                                            )                                                    ,                                  ⁢                  m          =          0                ,        1        ,        …        ⁢                                  ,                              2            ⁢                          N              RB              PDSCH                                -          1                                    [                  Equation          ⁢                                          ⁢          3                ]            
In Equation 3, NRBPDSCH represents the number of RBs corresponding to downlink data allocated to a specific UE. Therefore, according to the amount of downlink data allocated to a UE, the length of the sequence may vary.
A system evolving from an LTE system is referred to as a 3GPP LTE-Advanced (hereinafter, LTE-A) system. If a BS transmits an RS to a UE in the LTE-A system, a method for minimizing RS overhead while minimizing an influence on the existing LTE UE needs to be provided.